Formation of new blood vessels, caused by the overproduction of growth factors such as vascular endothelial growth factor (VEGF), is a key component of diseases like tumor growth, age-related macular degeneration (AMD) and proliferative diabetic retinopathy (PDR) (Connolly et al., J Clin Invest., 1989, 84(5):1470-8; Ferrara et al., Biochem Biophys Res Commun., 1989, 161(2):851-9; and Ferrara et al., Nat. Med., 1998, 4(3):336-40). Wet AMD is the most severe form of AMD disease that is characterized by abnormal neovascularization beneath the retina and often leads to permanent vision loss. Blocking of VEGF with antibodies, soluble VEGF receptors, or inhibition of VEGF receptor tyrosine kinase activity are strategies that have shown promising preclinical and clinical results in the suppression of retinal neovascularization (Aiello et al., PNAS, 1995, 92:10457-10461 and Willet, et al., Nat. Med., 2004, 10:145-147). However, recent clinical data shows that new vascular tissue typically does not regress with VEGF inhibition alone, because pericytes, which interact with endothelial cells and contribute to the establishment of the blood-retinal barrier, provide survival signals to neovascular endothelial cells and hence make them resistant to VEGF withdrawal (Benjamin et al., Development, 1998, 125(9)1591-8 and Patel S., Retina, 2009, 29(6 Suppl):S45-8). Furthermore, platelet-derived growth factor isoform B (PDGF-B) and PDGF receptor-beta (PDGFRβ), found in proliferative retinal membranes, have important roles in recruitment of pericytes for stabilization of the developing vasculature (Robbins et al., Invest Opth Vis Sci., 1994, 35(10):3649-63; Lindahl et al., Development, 1997, 124:3943-3953; and Hellström et al., Development, 1999, 126:3047-3055).
The VEGF binding function of VEGFR1 (Flt-1) has been mapped to the second extracellular domain (ECD) (Davis-Smyth et al., EMBO J., 1996, 15:4919-4927; Barleon et al., J Biol. Chem., 1997, 272:10382-10388; Wiesmann et al., Cell, 1997, 91:695-704; and Davis-Smyth et al., J Biol. Chem., 1998, 273:3216-3222). A naturally occurring alternatively spliced form of high affinity VEGF-binding receptor, soluble VEGFR1 (sFlt1), exists as a secreted protein that functions primarily as a decoy receptor (Shibuya et al., Oncogene, 1990, 5:519-524 and Kendall et al., PNAS, 1993, 90:10705-10709). A soluble receptor, VEGF-Trap, engineered for therapeutic use, has the second domain of VEGFR1 fused to the third domain of VEGFR2 (KDR) and to human IgG1 Fc region (Holash et al. 2002). An extracellular region of PDGFRβ was previously shown to antagonize PDGF-B stimulated responses (Duan et al., J Biol Chem, 1991, 266(1)413-8 and Ueno et al., Science, 1991, 252(5007):844-8). Studies with PDGFRβ-Fc chimera demonstrated that human PDGFRβ ECDs 1 to 3 are sufficient for high-affinity PDGF-B ligand binding (Heidaran et al., FASEB J., 1995, 9(1):140-5 and Lokker et al., J Biol Chem, 1997, 272(52):33037-44). An effect of predimerization on high-affinity PDGF-B ligand binding was also described when PDGFRβ ECDs 1 to 3 were fused to glutathione S-transferase (GST) domain (Leppänen et al., Biochemistry, 2000, 39(9):2370-5).
Current eye treatments require monthly intravitreal injections for years by a retinal specialist. Therefore, there is a need for improved therapeutic agents and an approach to deliver such therapeutic agents to sites such as the eye.